Quantum confinement ups optical-film efficiency

PORTLAND, Ore.  When he defined the Kuzyk Limit in 1999, Washington State University (Pullman) professor Mark Kuzyk demonstrated that optical materials could be made up to 30 times more sensitive than the state of the art at the time. Some progress has since been made toward that goal, but not enough for Kuzyk, who recently teamed with two WSU colleagues to publish design guidelines for creating highly light-sensitive "designer" molecules.

Now researchers in China and Belgium have answered the academics' call with an organic "chromophore" material that is said to be 50 percent more sensitive than anything previously tested.

Chromophores have been prime candidates for organic dye-sensitized solar cells, since they emit electrons when light is shone on them. But the new chromophore molecular configuration incorporates quantum confinement into each cell, greatly increasing chromophore efficiency.

The molecules were synthesized by chemist Yuxia Zhao at the Chinese Academy of Sciences (Beijing), evaluated by chemist Koen Clays at the University of Leuven (Belgium) and confirmed to be 50 percent more efficient by doctoral candidate Xavier Perez-Moreno at WSU.

"Your EE audience might be more interested in our application of imaging electric fields in a chip," said Kuzyk, who co-authored the design guidelines for light-sensitive molecules with WSU professor David Watkins and doctoral candidate Juefei Zhou. "You can coat a chip with an electro-optical polymer impregnated with dye molecules, and since the fraction of reflected polarized light is proportional to the electric field at that point in the circuit, you can effectively watch the gates turn on and off as the circuit is doing its thing."

Dye-sensitized solar cells were invented in the 1990s as an organic thin-film alternative to inorganic, silicon-based thin-film solar cells. In silicon solar cells, light is absorbed by an expensive semiconductor, but in dye-sensitized solar cells absorption occurs in an inexpensive thin film comprising dye molecules attached to titanium oxide nanoparticles in an electrolyte. When the dye cells absorb a photon, the resultant excitation injects electrons into the titanium, which transports them to the negative electrode.

Though dye-sensitized solar cells could potentially be manufactured very cheaply, thus far their low efficiencies have limited their widespread use. But when Zhao of the Chinese Academy of Sciences sent Leuven's Clays a series of seven molecules for test, Clays found that the quantum-defined material met Kuzyk's published design criteria. Perez-Moreno of WSU later confirmed that two of the seven showed 50 percent more efficiency than had ever been previously observed.